Various diversity techniques are known to achieve optimum results of demodulation by receiving radio waves with a plurality of antennas and combining them after having adjusted their phases and amplitudes individually. Patent literature 1 discloses, for instance, one of these techniques. There are also techniques of eliminating noises as other ways of improving sensitivity of the reception. One example of such techniques is to provide a second antenna for receiving a noise in addition to a first antenna for receiving the intended radio waves and to cancel out the noise by adjusting a phase and amplitude of the noise and combining it to a signal from the first antenna to achieve excellent reception. This technique is disclosed in patent literature 2 for instance. When used, this technique can reduce an influence of noise generated by the receiver itself upon the reception. It thus becomes possible to improve sensitivity of the reception even further by combining these two techniques and adding the process of eliminating the noise for each of the diversity antennas. FIG. 5 shows a conventional demodulation device representing one such example provided with four antennas for making diversity reception of terrestrial digital broadcasting waves while eliminating noises.
In FIG. 5, the demodulation device comprises antennas 10, 20, 30 and 40 for receiving radio waves of terrestrial digital broadcasting, noise collection probes 11, 21, 31 and 41, frequency converters 12, 13, 22, 23, 32, 33, 42 and 43, Automatic gain adjusters 14, 15, 24, 25, 34, 35, 44 and 45, Fourier transformers 16, 17, 26, 27, 36, 37, 46 and 47, noise eliminators 18, 28, 38 and 48, phase/amplitude modifiers 19, 29, 39 and 49, and combiner 1.
Antennas 10, 20, 30 and 40 receive radio waves that become potential noises generated by the receiver itself, besides the terrestrial digital broadcasting radio waves. Frequency converters 12, 22, 32 and 42 convert the radio waves received by antennas 10, 20, 30 and 40 into a predetermined frequency, and output them. Automatic gain adjusters 14, 24, 34 and 44 adjust gains of signals output from frequency converters 12, 22, 32 and 42 to bring them into a predetermined power level, and output them. Fourier transformers 16, 26, 36 and 46 perform Fourier transforms on the output signals of Automatic gain adjusters 14, 24, 34 and 44, and feed them as main signals to noise eliminators 18, 28, 38 and 48. On the other hand, noise collection probes 11, 21, 31 and 41 receive noises generated by the receiver itself. Frequency converters 13, 23, 33 and 43 convert noises received by noise collection probes 11, 21, 31 and 41 into the same predetermined frequency as frequency converters 12, 22, 32 and 42, and output them. Automatic gain adjusters 15, 25, 35 and 45 adjust gains of signals output from frequency converters 13, 23, 33 and 43 to bring them into a predetermined power level. Fourier transformers 17, 27, 37 and 47 feed the output signals of automatic gain adjusters 15, 25, 35 and 45 as noise signals to noise eliminators 18, 28, 38 and 48, after performing Fourier transforms on them. Noise eliminators 18, 28, 38 and 48 then adjust phases and amplitudes of the noise signals, eliminate noise signal components from the main signals, and output noise-eliminated signals. Phase/amplitude modifiers 19, 29, 39 and 49 modify the phases and amplitudes of the noise-eliminated signals according to commands of combiner 1, and output phase/amplitude modified signals. Combiner 1 combines the phase/amplitude modified signals output from four phase/amplitude modifiers 19, 29, 39 and 49, in addition to giving the command of phase value and amplitude level individually to the four phase/amplitude modifiers 19, 29, 39 and 49.
There is a drawback with the conventional demodulation device described above, however, that it requires as many number of noise eliminators as that of the antennas.    Patent Literature 1: Japanese Patent, No. 3,724,501    Patent Literature 2: Japanese Patent Unexamined Publication, No. 1982-111132